In recent years, a capsule endoscope appears in the field of endoscope. The capsule endoscope is equipped with an imaging function and a radio communication function. The capsule endoscope is swallowed by a subject for an observation (examination). After being swallowed, the capsule endoscope moves through inside internal organs (body cavities), such as a stomach and a small intestine, following a peristaltic motion inside the subject, and sequentially picks up images inside using the imaging function until being naturally discharged from a living body of the subject (human body).
While moving inside the internal organs, i.e., during an observation period, the capsule endoscope sequentially transmits data of the picked-up images of the body cavities to an external device placed outside the subject, using the radio function, e.g., by radio communication. The transmitted data is stored in a memory inside the external device. After swallowing the capsule endoscope, the subject carries the external device which has radio communication function and memory function until the capsule endoscope is discharged, whereby the subject can move freely. After the observation, a doctor or a nurse can retrieve the image data stored in the memory of the external device and watch images inside the body cavities on a monitor of a display device, for example, to make diagnosis (see Patent Document 1).
Some of the above described types of the capsule endoscopes are swallowable as described in Patent Document 1. One such proposed capsule endoscope includes a lead switch, which is turned on and off in response to an external magnetic field, in order to control driving of the capsule endoscope. The capsule endoscope is housed inside a package together with a permanent magnet which supplies the external magnetic field. The lead switch provided in the capsule endoscope remains in an off-state in the presence of a magnetic field of a predetermined strength, whereas the lead switch is turned on when the strength of the external magnetic field decreases. Hence, while housed inside the package, the capsule endoscope is not driven. When the subject is to swallow the capsule endoscope, he/she takes out the capsule endoscope from the package. Then, being taken away from the permanent magnet and freed from an influence of the magnetic force, the capsule endoscope starts to be driven. When the capsule endoscope has such a structure, the capsule endoscope is prevented from being driven while remaining inside the package. Once the capsule endoscope is taken out of the package, the capsule endoscope starts picking up images by the imaging function and transmitting image signals by the radio communication function.
Further, some capsule endoscopes as described in Patent Documents 2 and 3, for example, have imaging function and radio communication function similarly to the capsule endoscope of Patent Document 1. Once swallowed by the subject and placed inside the subject, the capsule endoscope advances inside the subject following peristaltic movements of alimentary tract. At the same time, the capsule endoscope picks up images at a fixed frame rate and radio transmits the intra-subject images. The intra-subject images as transmitted are received by a communication apparatus attached to the subject and recorded therein. After the capsule endoscope is discharged, the doctor or a nurse downloads the image data stored in the communication apparatus to a workstation, and makes diagnosis based on the intra-subject images displayed on a monitor of the workstation.
In the conventional capsule endoscopes, radio signals are transmitted only in one direction (uni-directional). Generally, the capsule endoscope only has a transmitting function, while the communication apparatus only has a receiving function. Regardless of a reception status of the communication apparatus, the capsule endoscope continues to radio transmit image signals once placed inside the subject.                Patent Document 1: International Publication WO 01/35813 pamphlet        Patent Document 2: Japanese Patent Application Laid-Open H1-305925 (Kokai)        Patent Document 3: Japanese Patent Application Laid-Open H4-109927 (Kokai)        